1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image containing a plurality of color components, i.e., a color image by forming color component images of different colors on corresponding image carriers and then transferring the color component images formed on the image carriers to a transfer medium. The invention particularly relates to an image forming apparatus which performs timing adjustment especially for registering the color component images.
2. Description of Related Art
As an apparatus for forming a color image on a paper, an image forming apparatus is known which individually forms color component images of e.g. black, cyan, magenta and yellow on corresponding photosensitive drums and then transferring the images to a transfer belt in a superimposed manner. In such an image forming apparatus, the forming of the color component images on the corresponding photosensitive drums is performed by reflecting laser beams outputted from a plurality of laser diodes by a plurality of polygon mirrors corresponding to the photosensitive drums to direct the laser beams to the photosensitive drums. Another type of image forming apparatus having a reduced number of polygon mirrors is also known in which laser beams from a plurality of laser diodes are directed to a common, i.e., a single polygon mirror, and the laser beams reflected by the single polygon mirror are directed to the corresponding photosensitive drums.
However, such an image forming apparatus has a problem that the image quality is degraded due to the misregistration of the color component images transferred to the transfer belt. Therefore, an image forming apparatus is known in which an image utilized for adjusting the timing of image forming (hereinafter referred to as “mark”) is formed and the position of the mark is detected to perform the timing adjustment based on the detected position (See Japanese Patent Application Laid-Open No. 4-149478 (1992), for example).
In the above-described image forming apparatus which uses a single polygon mirror, respective marks for the different color components, i.e. black, cyan, magenta and yellow, for example, are successively formed on a moving transfer belt, so that the timings at which the marks are formed differ among the color components. Therefore, the laser beams for the color components are not always reflected by the same surface of the polygon mirror, which poses a problem that the positions of the marks may deviate due to the individual difference among the surfaces of the polygon mirror. There is another problem that, in forming the marks of respective color components, the variation of the rotation speed of the belt driving roller for driving the transfer belt or the photosensitive drum causes the positional deviation of the marks.